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Geology and Mineralogy. 421 
through slate across the natural cleavage planes, the thermal con- 
ductivity diminishes with increase of temperature. 
(5.) We next tried a composite square slab of sandstone of the 
same dimensions as the slate, and we found for it also decisive 
proof of diminution of thermal conductivity with increase of tem- 
perature. We were not troubled by any cracking of the sand- 
stone, with its upper side kept cool by an ice-cold metal plate 
resting on it, and its lower side heated to probably as much as 
300° or 400° C. 
(6.) After that we made a composite piece, of two small slate 
columns, each 3°5 cm. square and 6:2 cm. high, with natural 
cleavage planes vertical, pressed together with thermoelectric 
junctions as before ; but with appliances (par. 10 below) for pre- 
venting loss or gain of heat across the vertical sides, which the 
smaller horizontal dimensions (7 cm., 3°5 em.) might require, but 
which were manifestly unnecessary with the larger horizontal 
dimensions (25 cm., 25 cm.) of the slabs of slate and sandstone 
used in our former experiments. The thermal flux lines in the 
former experiments on slate were perpendicular to the natural 
cleavage planes, but now, with the thermal flux lines parallel to 
the cleavage planes, we still find the same result, smaller thermal 
conductivity at the higher temperatures. Numerical results will 
be stated in par. 12 below. 
(7.) Our last experiments were made on a composite piece of 
Aberdeen granite, made up of two columns, each 6 cm. high and 
76 cm. square, pressed together, with appliances similar to those 
described in par. 6; and, as in all our previous experiments on 
slate and sandstone, we found less thermal conductivity at higher 
temperatures. The numerical results will be given in par. 12 
below. 
(8.) The accompanying diagram [here omitted] represents the 
thermal appliances and thermoelectric arrangement of pars. 6, 7. 
The columns of slate or granite were placed on supports in a bath 
of melted tin with about 0:2 cm. of their lower ends immersed. 
The top of each column was kept cool by mercury, and water 
changed once a minute, as described in par. 3 above, contained in 
a tank having the top of the stone column for its bottom and 
completed by four vertical metal walls fitted into grooves in the 
stone and made tight against wet mercury by marine glue. 
(9.) The temperatures, v(B), v(M), v(L) of B, M, T, the hot, 
intermediate, and cool points in the stone, were determined by 
equalizing to them successively the temperature of the mercury 
thermometer placed in the oil-tank, by aid of thermoelectric cir- 
cuits and a galvanometer used to test equality of temperature by 
nullity of current through its coil when placed in the proper cir- 
cuit, all as shown in the diagram. The steadiness of temperature 
in the stone was tested by keeping the temperature ot the ther- 
mometer constant, and observing the galvanometer reading for 
current when the junction in the oil-tank and one or other of the 
three junctions in the stone were placed in circuit. We also 
Am. Jour. Sc1—Tasrrp SERIES, Vou. L, No. 299.—NoveMBER, 1895. 
28 
